Linearly swept frequency generator



May 7, 1968 D. BLITZ ET AL 3,382,460

LINEARLY SWEPT FREQUENCY GENERATOR Original Filed May 9, 1966 5Sheets-Sheet 1 IO l2 R A M P I I T R O FR E OE EN CY CO LLED GENERATOROSCILLATOR SIGNAL I I I4 SLOPE PHASE FREQ PHASE CORRECTION 2O CIRCUTDETECTOR SAMPLING I FREQUENCY I7 GENERATOR F I G. l

I CYCLE 2 CYCLES 3 CYCLES 4 CYCLES m I /\I\ A M M III 91 v VI v vIIvvvIISAMPLING PULSE TIME AT TOR N EY May 7, 1968 I D. BLITZ ET AL 3,382,460

LINEARLY SWEPT FREQUENCY GENERATOR Original Filed May 9, 1966 5Sheets-Sheet 5 SAMPLING TIMES OUTPUT I40 |4b |4c H HOAFSE O H/ j H/ II Pl4d DETECTOR OUTPUT PULSE 55G STRETCHER 580-2 COMBINED OUTPUT OUTPUT OFOSC. l2

SAMPLE TIME DANIEL ETI T E F IG. 5 BY MARTIN R. c MoND ATTORNEY May 7,1968 D. BLITZ ET AL LINEARLY SWEPT FREQUENCY GENERATOR 5 Sheets-Sheet 4Original Filed May 9. 1966 OUTPUT OFJ I I I I MV H6 OUTPUT OF MVII8INVENTORS DANIEL BLITZ I I OUTPUT OFAJ I I I I OUTPUT OFl I I I I ISAMPLING SIGNAL MARTIN R. RIC BY ATTORNEY FIG.6

May 7, 1968 D. BLITZ ET 3,382,460

LINEARLY SWEPT FREQUENCY GENERATOR Original Filed May 9, 1966 5Sheets-Sheet 5 FROM TO DIFF. AMPLIFIER 52 NETWORK 59 FRoM PULSE GEN.|7

F I G. 7

FROM 4 AMPLIFIER G o INTEGRATOR I96 88 I90 F" 204 ll 7 I94 FRoM 0scII IATo INVENTQRS. DANIEL BLITZ MARTIN R. I MON!) ATTORNEY United StatesPatent 3,382,460 LINEARLY SWEPT FREQUENCY GENERATOR Daniel Blitz,Boston, and Martin R. Richmond, Belmont, Mass., assignors to SandersAssociates, Inc., Nashua, N.H., a corporation of Delaware Continuationof application Ser. No. 548,564, May 9, 1966. This application Sept. 11,1967, Ser. No. 667,334 20 Claims. (Cl. 331178) This invention relates toa sweep frequency generator. It relates more particularly to a generatorwhose output frequency changes precisely in accordance with apredetermined function of time.

The present application is a continuation of our presently pendingapplication, also entitled, Linearly Swept Frequency Generator, Ser. No.548,564, filed May 9, 1966, now abandoned.

A sweep frequency generator provides an output signal which sweeps overa given frequency range at a predetermined rate. Close control of thesweep rate is often highly desirable as a means of determining thefrequency at any time during the sweep. For example, a receiver arrangedfor surveillance of a frequency band may employ a sweep frequencygenerator as its local oscillator. Each received signal is timed fromthe beginning of the frequency sweep and the length of this intervalcorresponds to the frequency of the signal. Needless to say, theaccuracy of this method of frequency measurement depends on the accuracywith which the output of the sweep generator corresponds to itspredetermined time relationship.

As a general rule, once the generator commences its sweep, no provisionis made for detecting and correcting deviations from the desiredgenerator output signal. As a practical matter, these errors from theproper sweep can become quite large, particularly if the generator issweeping over a wide frequency range and uses an electronic frequencysweep mechanism for high speed operation.

In one instance of which we are aware, an attempt is made to control thegenerator output during the course of the sweep by measuring the time toreach specific frequency check-points in the sweep. This information isfed back to the generator so that if these times vary one way or theother from the known proper value, an error signal is applied to thegenerator to return the output signal to the proper frequency in thesweep. Even with this frequency spotting mode of sweep control, theaccuracy is not improved enough, particularly if a wide frequency rangeis involved. Moreover, the circuitry required for such frequencyspotting is quite complex and critical to adjust.

It is the principal object of this invention to provide a sweepfrequency generator whose output frequency can be precisely controlledin a predetermined way over a Wide frequency range.

Another object of this invention is to provide a sweep frequencygenerator capable of producing a linearly swept frequency signal havingaccurately controlled slope.

A still further object of this invention is to provide a controlledsweep frequency generator which is accurate yet which employs relativelysimple circuitry that is both stable and easy to adjust.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIG. 1 is a block diagram of a controlled sweep frequency generatorembodying the principles of this invention;

FIG. 2 is a graphical representation of the output of the generatorillustrated in FIG. 1;

FIG. 3 is a block diagram, partly in schematic form, of a linear sweepfrequency generator incorporating the invention;

FIG. 4 is a graphical representation of the outputs of various elementsof the circuit FIG. 3;

FIG. 5 is a graph showing corrections made to the output of thegenerator;

FIG. 6 is a graphical representation of the outputs of various elementsof the circuit illustrated in FIG. 3;

FIG. 7 is a more detailed schematic diagram of a portion of the circuitof FIG. 3, and

FIG. 8 is a similar detailed schematic diagram of another portion of thecircuit FIG. 3.

Briefly, our frequency generator employs a voltage controlled oscillatorwhich produces an output whose frequency sweeps through a givenfrequency range. The control voltage is varied so as to vary thefrequency of the output signal as a predetermined function of time overthe range. The phase of the output signal is sampled at intervals asdetermined by a sampling signal.

The sampling intervals are so chosen that the phase of the output signalchanges by predetermined increments between successive samplings.Generally, the intervals are selected so that the output signal containsonly integral numbers of cycles between successive samplings if itsfrequency varies at the prescribed rate. In other words, the signalwaveform gains (or loses) an integral number of cycles with eachsubsequent sample. Thus, the sampled phases will all be identical if thegenerator output frequency changes at the correct rate. Any variation inthe sampled phase from the norm is detected and fed back as an errorsignal to the voltage controlled oscillator to correct the slope, phaseand frequency of its output signal. In this sense, it is similar to aconventional phase lock circuit.

Although the invention has general application to sweep frequencygenerators producing output frequencies having many different functionsof time, it is particularly suitable for achieving a controlled linearfrequency sweep. In this case, the waveform gains (or loses) cycles at aconstant rate. Therefore, a constant sampling frequency can be used tocheck the phase of the continuously varying generator output frequency.For simplicity, the system is arranged to sample the output signal atzero axis crossings. As long as the average value of the signal duringthe brief sampling interval is zero, no error signal will be fed back tothe oscillator. However, if the signal has a leading or lagging phase,this will produce a finite average voltage which is used as an errorsignal to correct the output signal.

Referring now to FIG. 1, in its simplest form, a linear sweep frequencygenerator embodying the invention comprises a ramp generator 10 whoseoutput is fed to a volt age controlled oscillator 12. The linearlychanging output voltage from the ramp generator 10 causes the oscillator12 to sweep its frequency approximately linearly with respect to time inthe usual way. The output of the oscillator 12, which is also the outputof the frequency generator as a whole, is fed to a phase detector 14.

The phase detector 14 is turned on periodically by a signal from asampling frequency pulse generator 17.

Each time the phase detection 14 is pulsed on, it delivers an errorsignal which is indicative of the instantaneous phase of the oscillator12 at that moment. This error signal is fed to a correction circuit 20and there is processed to produce slope, phase and frequency correctionsignal components for the controlled oscillator 12.

Preferably, the correction signal components fed back to the voltagecontrolled oscillator 12 simultaneously correct the slope error ofthefrequency sweep and also reset the phase and frequency to where. theywould have been if the frequency. slope had been correct.

The selection of the correct sampling rate in relation to the frequencysweep is all-important to proper operation ofthe system. Preferably, thefrequency of the sampling frequency generator 17 (i.e. the rate at whichthe phase detector 14samples) is arranged so that the output of thevoltage controlled oscillator 12 contains an integral number of cyclesin the interval between each sampling pulse and the next. Therefore, ifthe frequency sweep is correct, the sweep signal has the very same phaseeach time it is sampled, even though its frequency changes betweensamples. While sampling at any phase angle is feasible, it is mostconvenient to sample the swept frequencysignal at a zero axis crossingwhen its amplitude is zero, for then the sampled phase, if correct, willbe reflected by a zero output from phase detector 14.

In the case of a linear sweep frequency generator, the samplingintervals can be uniform and consequently can be controlledby a samplingfrequency generator 17 having a constant output frequency, i.e. a clock.The sampling rate is then determined in the following way.

With a linear sweep, the frequency of the output signal from the voltagecontrolled oscillator 12 may be expressed by the following relationship:

where f is the frequency (cycles/second); f is the initial frequency ofthe sweep (at 1:0);

is the rate of change of frequency (cycles/second and t is the timefromstart of sweep (seconds).

The phase of the signal is given by:

0=ffdt=0+f t+V2 6t where?) is the phase in cycles; and 0 is the phase att=0.

where n is the sample number at time t (e.g. 12:0); oc-

curs at t=0);

At is the time increment between samplings; and

i is the sampling frequency=1/At.

Therefore, the phase of the nth sample is IQ'I L 6n f5 ft It follows,then,,that the phase change A0 between sucwhere k is the number ofadditional cycles to be added between successive samplings; and c is theinitial number of cycles between samplings.

Solving Equations 6 and 7 for the sampling frequency f and the initialfrequency of the sweep f it is seen that and Also, since the phase angleis preferably an integral multiple of a cycle between samples, as notedabove, it follows from Equation 8 that k and 0 should both be integers.

In the case of a linear sweep, in which the output signal fromoscillator 12 adds one additional cycle be tween successive samplings,k=1. In this case, from Equations 9 and 10, it is seen that:

FIG. 2 illustrates graphically the relationship which obtains betweenthe frequency sweep of the oscillator 12 and the output of the samplingfrequency generator for the above example. The sampling pulses derivedby generator 17 occur at uniformly spaced intervals. However, the signalfrom the oscillator 12 is continuously increasing in frequency betweensamples. Nevertheless, each sweep interval between samples contains anintegral number of cycles, there being one additional cycle betweensuccessive samples. Moreover, the phase of the sweep signal at the timeof sampling is always the same, to wit: zero degrees in this example.

If it is desired to sample every half cycle, it is apparent from FIG. 2that the phase of the sampled signal will change by one-half cycle witheach successive sample with a corresponding alternation in the polarityof the output of the phase detector 14. This phase reversal betweensuccessive samplings can be countered by continually reversing thepolarity of the input or output signal of the phase detector. However,this complicates the circuitry. Therefore, to avoid this problem, it isdesirable, as noted above, to have an integral number of cycles betweensamples (i.e. make both k and c integers).

In the case of the linear sweep, polarity reversal between samples canbe avoided also by increasing theinterval between samplings, i.e. makek=2.. From Equation 9,

Now, however, the sampling frequency (i is only the square root ofone-half as great as before, resulting in a possible sacrifice in thetightness of the feedback control of the voltage controlled oscillator12.

It may be desirable to start the frequency sweep at zero frequency andincrease the interval between samplings by one cycle (i.e. maintaink=1). In this event, from Equations 9 and 10 i and 0 cycle From Equation15, it is seen that the phase of the sampled signal will change byone-half cycle with each successive sample, with a correspondingalternation in the polarity of the output of the phase detector 14. Thisphase reversal can be countered by reversal polarity as described above.

It should also be understood that sweeps other than linear ones can becontrolled using the technique described herein. For example, the sweptfrequency signal from oscillator 12 may vary as the square of time,i.e.:

f= .+fo

By following the same type of derivation as before, it is found thatwhere p is a constant factor of additional cycles to be added betweensuccessive samplings, and q is the initial number of cycles betweensamplings.

In this case, the sampling frequency i and the initial frequency f ofthe swept signal are given by Also, the sampling frequency i and initialfrequency t are given by 1 t m (p Sampling should occur when f(n) inEquation 21 is an integer. It should be emphasized at this point that iis not a function of time, but is constant, as seen from Equation 25.

Thus, it is a very significant aspect of this invention that despite thenonlinearity of the desired frequency sweep. the sampling frequency (iat which the swept signal is ultimately controlled can be constant withtime and therefore can be generated by a simple fixed frequency clock.

Other complex sweeps can be controlled by programming the samplingfrequency to change at the appropriate rate to leave an integral numberof cycles of the swept signal between each sample. For example, two ofthe described controlled sweep frequency generators may be connected incascade so that the swept output signal from the first determines therate of sampling in the second generator.

Refer now to FIG. 3 which illustrates in greater detail a controlledsweep frequency generator having a linearly swept output. The generatoris basically the same as that illustrated in FIG. 1. Preferably theinitial frequency, prior to the beginning of the sweep, is accuratelycontrolled by means of a phase locking arrangement using a multiple ofthe frequency of the sampling frequency generator 17 as a reference.

As shown in FIG. 3, the ramp generator may be an integrator employing anoperational amplifier 30 having input and output terminals 34 and 36,respectively. Terminal 34 is connected through a pair of seriesresistors 38 and 40 to a voltage source illustrated by a battery 37.

and

The output of amplifier 30 is fed back to input terminal 34 via acapacitor 42. A resistor 44 included in the feedback network in parallelwith capacitor 42 causes the ramp generator 10 to integrate slightlynonlinearly to compensate for nonlinearity in the voltage controlledoscillator 12.

A switch S1, connected to short circuit the input voltage from thebattery 37, and a switch 82, connected to discharge the feedbackcapacitor 42, are closed to reset the ramp generator 10 and then openedto initiate generation of its output signal. This signal, which is avoltage increasing approximately linearly with time, is fed via a seriesresistor 46 to the control terminal 47 of the oscillator 12.

The output of the oscillator 12 is also fed to a mixer 48 where it ismixed with the output of a crystal controlled oscillator 50. The lowfrequency output of the mixer 48 is then passed via a conventional wideband amplifier 52 to a frequency sweep control circuit indicatedgenerally at'53, which ultimately controls the frequency of oscillator12 during the actual sweep or ramp period. The amplified signal frommixer 48 is also fed to a constant frequency control circuit indicatedgenerally at 54 which phase locks the output signal from oscillator 12to the system reference frequency prior to the start of the frequencysweep. It should be mentioned at this point that the frequency controlcircuits 53 and 54 actually sample and synchronize theoutput of themixer 48 rather than the output of oscillator 12. They could, however,sample the oscillator 12 output directly. Accordingly, the mixer 48 andfixed frequency oscillator 50 are to be treated herein as part of thevoltage controlled oscillator 12.1

The frequency sweep control circuit includes the phase detector 14,which receives the output of amplifier 52 and compares its phase withthat of the output of a frequency divider 56 in the sampling frequencygenerator 17. The output frequency of the divider 56 is preferably inthe form of pulses occurring at a submultip'le of the frequency of areference frequency oscillator 57 used as a phase reference in theconstant frequency control circuit 54. Detector 14 is switched onpediodically at the sampling rate and, as noted above, its outputreflects the phase error in the signal from oscillator 12 at the time ofsampling. This error signal, which has the form of a series of briefpulses, is fed to a modified pulse stretcher 55.

The output from the pulse stretcher '55 is a continuous voltage whoselevel increases or decreases in correspondence with successive errorpulses from phase detector 14. A positive pulse causes the level tobecome more positive, While a negative pulse causes it to change in thenegative direction. If there is no phase error in the output of theoscillator 12, the output of the pulse stretcher 55 remains unchanged.These relationships are illustrated in FIG. 4.

The signal from pulse stretcher 55 is processed three ways. First, it isused directly as a frequency correction. Second, it is integrated by amodified integrating network indicated generally at 58 to obtain a slopecorrection. Finally, it is differentiated by the network indicatedgenorally at 59 to establish a phase correction. Alternatively, thepulses from the phase detector 14 might be used directly for phasecorrection. All of these correction signal components are fed back tothe input of the voltage controlled oscillator -12 so that ideallyduring each sample, they correct the sweep slope error and reset thephase and frequency to where they would have been if there had been noslope err-or.

FIG. 5 illustrates the reason for correcting all three characteristicsof the oscillator signal. Ideally, the frequency of the oscillator 12 isto change linearly with time in accordance with the curve 12a. However,it may depart from the desired slope and follow the curve 12b(exaggerated slope). This results in a departure from the phasesynchronism illustrated in FIG. 2' and the phase detector 14 *(FIG. 3)emits a corresponding error signal at the time of the next sampling. Aslope corre ction signal from the modified integrator 58 returning theoscillator 12 to the correct slope will then provide operation along acurve 120. However, there will then be a frequency error in theoscillator signal corresponding to the vertical distance between thecurves 12a and 120. A frequency correction derived from the error signalwill jump the frequency. of the oscillator by the amount of thisfrequency error, as indicated at 12d, so as to return the oscillatorsignal to the curve 12a.

The oscillator output signal will now be in synchronism with thesampling frequency signal. However, as a result of the phase changeoccasioned by the departure along the curve 121), the axis crossings ofthe sweep signal will no longer occur at the time of future samplings.Rather, the successive samplings will see some other points in theoscillator Waveform. Accordingly, the phase detector 14 will indicate anerror even though the oscillator frequency ,now has the desired value.The phase correction derived from the original error signal by means ofthe differentiating network 59 has the form of a pulse which momentarilyshifts the frequency of the oscillator 12 so as to change the phase ofits output signal and give it the value it would have had if the signalhad originally continued along the curve 12a. That is, with the phasecorrection signal, the desired zero axis crossing again occurs at thesampling time. Consequently, immediately after detection of a samplingerror, not only is the slope error corrected, but also the sweepfrequency is reset to the frequency and phase it would have had if therehad been no slope errorinitially.

It should be noted that oscillator correction satisfactory for manyapplications can be obtained solely from slope correction or frequencycorrection. However, the speed of correction and accuracy of control areoptimized by using all three types of correction. Moreover, if slopecorrection is to be used, frequency and phase correction are availablewith only a very slight increase in the number of circuit components.

The differentiating network 59 comprises a capacitor 60 and resistor 62conected in series between pulse stretcher 55 and network 58. A secondresistor 64 is connected in parallel across the resistor-capacitorcombination to pass the steady-state components of the signals from thepulse stretcher. The integrating network 58 comprises simplyaconventional operational amplifier 68 with an input terminal 69conected to network 59. The amplifier output terminal 72 is connectedvia a series resistor 78 to the control terminal 47 of the voltagecontrolled oscillator 12. A resistor 74 and capacitor 76 constitute afeedback network between output terminal 72 and input terminal 69. Aswitch S3 is connected across capacitor 76 to discharge the capacitor 76upon completion of each frequency sweep.

At the beginning of each frequency sweep, the voltage across thecapacitor 76 is zero, and preferably the output voltage of the pulsestretcher 55 and the integrator 48 are also zero. Next, assume as inFIG. 4 that the phase detector 14 emits an error-indicating pulse 14afollowed by a corresponding output level 55a from the pulse stretcher.This results in a signal at the output of the integrator 58 containingcomponents for all three of the desired oscillator corrections.

More specifically, the increase in pulse stretcher output voltage to thelevel 55a (FIG. 4) results in a phase-correcting pulse 59a from thedifferentiating circuit 59. This pulse is passed through to the outputterminal of the integrator 58 by way of the feedback network in theintegrator. To this end it is desirable that the feedback network have asubstantially greater time constant than the differentiating circuit 59,by virtue of a relatively large capacitance in the capacitor 76.

At the same time, the characteristics of the amplifier the constantoutput'voltage of the pulse stretcher charges the capacitor 76 linearlywith time so that the output voltage of the integrator has acorrespondingly linear increase. This component, illustrated at 58a2 inFIG. 4 is the slope correction signal for the oscillator 12.

As indicated in FIG. 4, the correction signal components 59a, 58a1 and58a-2 alter the output signal of the oscillator 12 sufficiently to keepthe output of the mixer 48 in synchronism with the reference signalthrough the next sampling period14b. Thus, there is no output from thephase detector 14 at this time and the frequency and slope correctioncomponents 58a1 and 58a-2 continue unchanged.

Another error signal14c appears during the succeeding sampling interval,however, with a resultant change in pulse stretcher output to the level550. In the manner described above, this results in a phase correctingpulse 590, an additional frequency correcting component 580-1 and achange in the slope correcting component as indicated at 58c2.

During the next sampling interval, an error signal 14d, of thesamemagnitude but opposite polarity to the signal 14c, restores theoutput of the pulse stretcher 55 to its previous level as indicated at55a. This results in a phase correcting pulse 59d together withcorresponding changes in the frequency and slope-correcting componentsas indicated at 58d1 and 58d2.

With reference to FIG. 3, as mentioned previously the sweep frequencygenerator also includes the constant frequency control circuit 54 whichsynchronizes the initial frequency of the oscillator 12 to the output ofthe reference frequency oscillator 57 in a conventional phase lockarrangement, prior to the start of the frequency sweep. Control circuit54 comprises a switch 82 which passes the output of amplifier 52 to acombination discriminator and phase detector 84.

.The discriminator and phase detector 84 receives a sampling frequencyfrom the reference oscillator 57 and provides an error signal indicativeof the difference in frequency and phase between the outputs of theamplifier 52 and oscillator 57. The error signal, in turn, is integratedby an integrator 88 and the output of the integrator is applied to theoscillator 12 control terminal 47 by way of a resistor 98 to close thecontrol loop.

The resistors 46, 78 and 98 combine with the input resistance of theoscillator 12 to form a summing network which sums the output voltagesof the ramp generator 10 and the integrators 58 and 88.

Still referring to FIG. 3, a timing network indicated generally at 99controls the commencement and. termination of the frequency sweep of thevoltage controlled oscillator 12. FIG. 6 shows the timing sequenceinvolving the network 99 and reference to this figure during thefollowing discussion will facilitate an understanding of the operationof the network. The network 99 comprises a multivibrator 116 whichinitiates operation by generating a square pulse whose leading edgesimultaneously triggers a pair of one shot multivibrators 118 and andsets a flip-flop 126. The flip-flop thus closes the switch S1 tointerrupt the input voltage for the ramp generator 10. In its unstablestate the multivibrator 118 provides a voltage at its output terminal119 to close the switches S2 and S3 in the ramp generator 10 andintegrator 58. This resets both devices by discharging the capacitors 42and 76. As seen in FIG. 6, it remains in its unstable state for only ashort time.

The one shot multivibrator 120 controls the switching between. thefrequency sweep and constant frequency control modes. On being triggeredby the multivibrator 116, it opens switch 114 and closes switch 82. Thisstops the operation of frequency sweep control circuit 53 bydisconnecting the phase detector 14 from sampling frequency generator17, and enables operation of the constant frequency control circuit 54in the manner described above with the discriminator and phase detector84 receiving signals 57a from amplifier 52 as well as directly from thereference oscillator 57.

The multivibrator 120 preferably has a somewhat slower return time thanthe multivibrator 118. When it returns to its stable state, it opens theswitch 82, thereby disabling the constant frequency control circuit 54.At the same time, it closes switch 114 to initiate the frequency sweepand also enable operation of the frequency sweep control circuit 53.

Specifically, the switch 114 now passes the sampling frequency signal17a from the sampling generator 17 to the phase detector 14. This signalis also coupled via switch 114 to the reset input of flip-flop 126. Asseen in FIG. 6, the first such signal 17a after the return ofmultivibrator 120 resets flip-flop 126. This, in turn, opens the switchS1 to initiate ramp generation by the ramp generator 10.

Accordingly, at this point, the ramp generator 10 and frequency sweepcontrol circuit 53 have been reset. The constant frequency controlcircuit 54, having synchrnized the initial frequency of the sweep to thefrequency f of oscillator 57, as seen from the lowermost curve in FIG.6, is disabled, but it nevertheless holds its frequency-correctingsignal. Finally, the lower rate (f sampling pulses from generator 17 arenow being fed to phase detector 14. The frequency sweep begins with thefirst such pulse 17a.

The output terminal 127 of flip-flop 126 is also connected via acapacitor 130, a resistor 132, a potentiometer 134 and a summingresistor 136 to control terminal 47 of the oscillator 12. Theresistor-capacitor combination forms a differentiating network whichpasses a pulse to the oscillator 12 upon the resetting of flip-flop 126at the beginning of the frequency sweep. The pulse causes a step oroffset 139 (FIG. 6) in the initial frequency f of the oscillator fromthe value corresponding to the frequency f of the oscillator 57. Thereason for this offset 139 is discussed below.

The frequency sweep continues until the leading edge of the next pulsefrom multivibrator 116 is applied to the multivibrators 118 and 120 andflip-flop 126. This returns the oscillator 12 to its initial frequencyand switches the control system to the constant frequency mode toprepare for another frequency sweep as described above.

As mentioned previously, it is desirable to have an integral number ofcycles between samples during the frequency sweep. Also, switching backand forth between the constant frequency and frequency sweep modes asdescribed above make it desirable to use reference signals during thetwo modes which are synchronized to each other. The simplest way toaccomplish this is by using the frequency divider 56 (FIG 3). As aresult, the sampling rate f during the sweep is an integral submultipleof the frequency f of the reference oscillator 57 so that the samegenerator can be used to control frequency during both the frequencysweep and constant frequency portions of the operating cycle.

As noted above, the first of these conditions is satisfied when k and care both integers. The second condition may be expressed as Therefore,in order to make f the initial frequency of the sweep, equal to f thereference frequency to which the oscillator 12 is set during theconstant frequency mode,

must equal the integer 111. Since k and c are both integers, it isapparent that f cannot equal f unless it is an even number. However, asnoted previously, increasing the number of cycles between samples, i.e.making k l, makes for less control over the frequency sweep.

The generator illustrated in FIG. 3 ties the initial frequency of thesweep to the frequency of oscillator 57, while still maintaining thehighest practical sampling rate (i.e. with k l) by introducing thefrequency offset 139 (FIG. 6) at the beginning of the sweep.Specifically, with k: 1, Equation 10 may be rewritten as From (10a) f isobviously not an integral multiple of f and therefore f cannot equal fgiven the foregoing conditions.

However, if

then f will be an integral multiple of f This is accomplished by meansof the offset 139, which reduces the frequency of the oscillator 12 byf,/ 2 so that a condition indentical with Equation 29.

The heterodyne arrangement involving the mixer 48 and oscillator 50 isvery useful in this connection. The quantities f (at the output of themixer 48) and i can be selected with the foregoing constraints in mindand the desired frequencies of the oscillator 12 can then be obtainedmerely by selecting the corresponding frequency for the oscillator 50.That is, with the FIG. 3 circuit, changing the frequency of oscillator50 up or down will move the sweep frequency range up or downcorrespondingly. It should be emphasized that the frequency f of thesignal from mixer 48, which is synchronized and sampled as describedabove, remains the same. That is, the control circuits 53 and 54 will inany case force the output frequency of the mixer 48 to conform to theoutputs of the pulse generator 17 and oscillator 57 in the mannerdescribed.

While we have specifically shown a heterodyne arrangement which beatsdown the oscillator 12 frequency, the oscillator can also operate at lowfrequencies. Then, however, sampling would normally occur at widelyspaced time intervals. To avoid this, the system can then beat thatfrequency up and sample the higher frequency so that the sampling ratewill be high enough to maintain tight control over the output frequency.

In another variation of the system, the output of oscillator 12 may besampled directly and the output of the generator as a whole taken fromthe mixer 48.

Refer now to FIG. 7, which shows in greater detail the phase detector 14and pulse stretcher 55 employed in the sweep frequency generatorillustrated in FIG. 3. The phase detector 14 comprises four diodes 140,142, 144 and 146 connected together to form a conventional diode bridgeswitch. The A.C. signal from amplifier 52 is coupled via a capacitor 148to the junction of diodes and 142. Sampling pulses from the pulsegenerator 17 are applied between the junction of diodes 140 and 146 andthe junction of diodes 142 and 144. Each such pulse switches on thediode bridge switch momentarily so that the signal from the amplifier 52appears at the junction 1 1 149 of diodes 144 and 146 which constitutesthe output terminal of the phase detector 14. If the fequency sweep iscorrect, the zero axis crossings of thesignal from amplifier 52 coincidewith the sampling pulses from generator 17, and the voltage at theoutput terminal 149 of the phase detector is zero. If, however, there isan error in the frequency sweep, each time the detector 14 is switchedon, an appropriate error voltage appears at the terminal 149.

The output terminal149 of thephase detector 14 is connected to pulsestretcher 55 which comprises a capacitor152 connectedbetween terminal149 and ground. An error pulse from phase detector 14 appears as avoltage level across capacitor 152 (as shown in FIG. 4). The signal onthe capacitor is then simplifiedby an amplifier 154 and coupled to thedifferentiating network 59. The pulse stretcher 55 also includes afeedback network indicated generally at 156 which feeds the output ofpulse stretcher 55 to the junction of diodes 140 and 142. This networkcomprises a resistor 158, a series connected choke 160 and a capacitor162 connected between ground and the junction of the resistor and choke.The resistor 158 and the output resistance of amplifier 52 (FIG. 3) forma summing network which sums the outputs of the amplifier and pulsestretcher. Therefore, the signal impressed on detection mode ofoperation, which is elfective when the frequency from the amplifier 52is close to the reference frequency and thus the error signal fromdiscriminator operation is relatively small. The relatively large errorsignal from the phase detector mode of operation provides fine frequencycontrol bringing the output of the amplifier 52 into synchronism withthe reference signal from the oscillator 57.

It will be seen from the foregoing that our sweep frequency generatoraccurately controls the swept signal over the entire frequency sweep.With this invention, a precisely controlled linear sweep limited only bythe characteristics of the components is possible, as Well as controlledsweeps which increase or decrease with some other function of time. Asystem calling for a sweep up to mc. is currently in operation.Moreover, close control of the sweep is achieved without unduecomplication of the circuitry ascompared with prior systems.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding descrip tion, are efiiciently attained and,since certain changes may be made in the above construction withoutdeparting capacitor 152 during a given pulse from generator 17 comprisesthe signal on the capacitor at end of the previous pulse plus anyadditional error voltage due to the I incoming signal from amplifier 52.In this way, the pulse stretcher holds its error voltage from onesampling to the next producing the continuous stepped D.C. error signal,described above, throughout the frequency sweep. The feedback network156 provides a delay in feedback to prevent summing of the feedbacksignal produced during a given pulse from generator 17 until the arrivalof lel with the capacitor 152. The switch S4 may then be controlled bymultivibrator .120 (FIG. 3).

FIG. 8 illustrates in greater detail the phase shift discriminator andphase detector 84 employed in the FIG.

3 circuit. The signal from amplifier 52 is applied to the primarywinding 172 of a transformer 174. The transformer secondary winding 176is connected to a diode bridge indicated generally at 178 whichcomprises the usual four diodes 180,182, 184 and 186. More specifically,one end of the transformer secondary winding 176 is connected viaresistor 188 to the junction of diodes 180 and 186. The other end of thetransformer secondary winding is connected by way of resistor 190 tothejunction of diodes 182 and 184.

A resistor 192 and choke 194 are connected in series between ground andthe junction of diodes 180 and 182.

The resistor serves as a termination for a line from the oscillator 57;the choke provides isolation for signals from the amplifier 52. An R-Cnetwork indicated generally at 196 connected between amplifier 52 andthe junction of diodes 180 and 182 provides a 90 phase shift at thereference frequency f Thus the diode bridge 178, transformer 174 andnetwork 196 operate together as a conventional discriminator whichapplies to a load resistor 204 an error signal indicative of thedifference between the reference frequency and the output frequency ofthe amplifier 52. This signal serves for coarse frequency control in thecontrol circuit 54 ofFIG. 3.

At the same time, the signal from reference frequency oscillator 57 isapplied to the junction of resistor 192 and choke 194. This signaloperates the bridge in a phase from the scope of the invention, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire tosecure by Letters Patent is:

1. A controlled sweep frequency generator for providing an output whosefrequency varies as a selecte function of time, said generatorcomprising A. a variable frequency generator,

-B. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude and polarity correspond to the difference between said phaseand the phase of a reference signal,

C. a sampling frequency generator for controlling the sampling rate ofsaid sampling means, said sampling frequency generator having afrequency such that when the output signal from said variable frequencygenerator changes frequency as said function of time, the phase of saidoutput signal changes by predetermined increments between successivesamplings by said sampling means, and

D. means for correcting the frequency of said variable frequencygenerator by applying to said variable frequency generator a correctionsignal in response to said error signal so as to make the variation ofsaid frequency conform substantially to said function of time.

2. A controlled sweep frequency generator as defined in claim 1 whereinsaid sampling frequency generator has a frequency such that so long assaid output signal from said variable frequency generator changes assaid given function of time, an integral number of cycles of said outputsignal falls between successive samplings.

. 3. A controlled sweep frequency generator as defined in claim 1wherein said correction signal includes slope, phase and frequencycorrection components.

4. A controlled sweep frequency generator as defined in claim .1 andfurther including a waveform generator for applying a time-varyingvoltage to said variable frequency generator so as to cause saidfrequency to vary approximately as said function of time, saidcontrolling means controlling said variable frequency generator tocorrect deviations in the variation of said frequency from said selectedtime function.

5. A controlled sweep frequency generator as defined in claim 4 whereinsaid phase changes by equal increments between samplings.

6. A controlled sweep frequency generator as defined in claim 4 whereinA. the sampling frequency generator has a constant output frequency, andB. said function of time is a power of time.

7. A controlled sweep frequency generator as defined in claim 6 whereinsaid function of time is linear.

8. A controlled sweep frequency generator as defined in claim 4 whereinsaid samplings occur at zero axis crossings of said output signal fromsaid variable frequency generator when said output signal varies as saidfunction of time.

9. A controlled sweep frequency generator as defined in claim 4 andfurther including A. a reference oscillator, and

B. means for synchronizing the output of said variable frequencygenerator to the output of said reference oscillator prior to thebeginning of the frequency sweep.

10. A controlled sweep frequency generator as defined in claim 9 whereinthe frequency of said sampling frequency .generator is an integralsubmultiple of the frequency of said reference oscillator.

11. A controlled sweep frequency generator for providing an output whosefrequency sweeps as a selected function of time, said generatorcomprising A. a variable frequency generator,

B. a waveform generator for applying a control voltage to said variablefrequency generator so as to cause said frequency to sweep approximatelyas said function of time,

C. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude and polarity correspond to the difference between said phaseand the phase of a reference signal,

D. a sampling frequency generator for controlling the sampling rate ofsaid sampling means, said sampling frequency generator having afrequency such that when the output from said variable frequencygenerator sweeps frequency as said function of time, the phase of saidoutput signal changes by equal increments between samplings, and

E. a correction circuit for correcting the frequency of said variablefrequency generator by applying a correction signal thereto in responseto said error signals so as to make said frequency sweep in substantialconformity with said function of time, said correction circuit includingmeans for producing a continuous output voltage whose level changes incorrespondence with said error signals, said voltage levels beingfrequency correction components in said correction signal.

12. A controlled sweep frequency generator as defined in claim 11wherein said correction circuit also includes means for generating atime-varying voltage during each sampling whose time variation isproportional to the sum of said error signals from said sampling meansduring said frequency sweep, said time-varying voltage being a slopecorrection component in said correction signal.

13. A controlled sweep frequency generator as defined in claim 11including means for coupling to said variable frequency generator duringeach sampling a pulse proportional to the phase error in said frequencysweep, said pulse being a phase correction component in said correctionsignal.

'14. A linear sweep frequency generator whose frequency sweeps over aselected range comprising A. a variable frequency generator.

B. a waveform generator for applying a time-varying voltage to saidvariable frequency generator so as to cause said frequency to sweepapproximately linearly,

C. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude and polarity correspond to the difference between said phaseand the phase of a reference signal,

D. a sampling frequency generator for controlling the sampling rate ofsaid sampling means, said sampling frequency generator having afrequency such that when the frequency of the output signal from saidvariable frequency generator changes linearly, the phase of said outputsignal changes by equal increments between samplings, and

E. a correction circuit responsive to said error signals for correctingthe frequency of said variable frequency oscillator by applying theretoa correction signal so as to make the frequency of said oscillator sweeplinearly, said correction circuit including (1) means for producing acontinuous output voltage whose level changes in correspondence withsuccessive error signals during successive samplings by said samplingmeans, said voltage levels being frequency correction components in saidcorrection signal,

(2) means for generating a ramp voltage during each said sampling whoseslope is proportional to the sum of said error signals from saidsampling means, said ramp voltages being slope correction components insaid correction signal,

(3) means for coupling to said variable frequency generator during eachsampling a pulse proportional to the phase error in said output signal,said pulses being phase correction components in said correction signal.

15. A linear sweep frequency generator as defined in claim 14 andfurther including A. means for removing said correction signal from saidcorrection circuit and resetting said variable frequenc y generator whenthe frequency of said output signal reaches the end of said range.

16. A controlled sweep frequency generator for providing an outputingnal whose frequency sweeps as :a selected function of time, saidgenerator comprising A. a variable frequency generator,

B. means for controlling said variable frequency generator so that itsweeps frequency approximately as said selected function of time,

C. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude "and polarity correspond to the difference between said phaseand the phase of a reference signal,

D. a sampling frequency generator for controlling the sampling rate ofsaid sampling means,said sampling frequency generator having a frequencysuch that when the output signal from said variable frequency generatorsweeps frequency as said function of time, the phase of said outputsignal changes by predetermined increments between successive samplingsby said sampling means,

E. a correction circuit responsive to said error signals for correctingthe frequency of said variable frequency generator by applying thereto acorrection signal to make the variation of said frequency conformsubstantially to said function of time,

F. :a reference oscillator whose signal frequency is an integralmultiple of the frequency of said sampling frequency generator, and

G. a phase lock circuit connected to develop a control voltagesynchronizing the output of said variable frequency generator to theoutput of said reference oscillator prior to the beginning of saidfrequency sweep.

17. A controlled sweep frequency generator as defined in claim 16wherein said phase lock circuit holds its said control voltage duringsaid frequency sweep.

18. A controlled sweep frequency generator as defined in claim 16 andfurther including means for offsetting the frequency of said variablefrequency generator from that of said reference oscillator at thebeginning of said frequency sweep so as to adjust the phase of saidswept signal by a predetermined amount.

19. A linear sweep frequency generator which sweeps over a selectedfrequency range, said. generator comprising A. a variable frequencygenerator,

B. a waveform generator for applying a ramp voltage to said variablefrequency generator so as to cause said frequency to sweep linearly,

C. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude and polarity correspond to the difference between said phaseand the phase of a reference signal,

D. a sampling frequency generator for controlling the sampling rate ofsaid sampling means, said sampling frequency generator having afrequency such that when the output signal from said variable frequencygenerator sweeps frequency linearly, the phase of said output signalchanges by an integral number of cycles between samplings, t

E. a correction circuit for correcting the frequency of said variablefrequency generator by applying a correction signal thereto in responseto said error signals so as to make said frequency sweep substantiallylinearly, i

F. a reference oscillator whose frequency is an integral multiple ofsaid frequency of said sampling frequency generator, I

G. a timing network,

H. means responsive to a first signal from said timing network forresetting said variable frequency oscillator frequency to the beginningof said frequency range after the swept signal reaches the end of itssaid range,

I. means responsive to a second signal from said timing network forremoving said correction signal from said correction circuit prior tothe beginning of the frequency sweep,

J. a constant frequency control circuit responsive to a third signalfrom said timing network for synchronizing the initial output of saidvariable frequency generator to the output of said reference oscillatorprior to the beginning of the frequency sweep by developing errorsignals proportional to the phase difference between said output signalfrom said variable fre quency generator and said reference oscillatoroutput, said timing network also controlling said waveform generator soas to commence the frequency sweep after the output of said variablefrequency generator has been synchronized to the output of saidreference oscillator.

20. A linear sweep frequency generator whose frequency sweeps over aselected frequency range comprismg i A. a variable frequency generator,

B. a waveform generator for applying a time-varying voltage to saidvariable frequency generator so as to cause said frequency to sweepapproximately linearly,

C. means for repetitively sampling the phase of the output signal fromsaid variable frequency generator to develop error signals whosemagnitude and polarity correspond to the difference between said phaseand the phase of a reference signal,

D. a sampling frequency generator for controlling the sampling rate ofsaid sampling means, said sampling frequency generator having afrequency such that when the frequency of the output signal from saidvariable frequency generator changes linearly, the phase of said outputsignal changes by an integral number of cycles between samplings,

E. a correction circuit responsive to said error signals for correctingthe frequency of said variable frequency generator by applying thereto acorrection signal so as to make the frequency of said variable frequencygenerator sweep substantially linearly, said correction circuitincluding (1) means for producing a continuous output voltage whoselevel changes in correspondence with successive error signals, saidvoltage level being a frequency correction component in said correctionsignal,

(2) means for generating a ramp voltage during each said sampling whoseslope is proportional to the sum of said error signals from saidsampling means during the frequency sweep, said ramp voltage being aslope correction component in said correction signal,

(3) means for coupling to said variable frequency generator during eachsampling a pulse proportional .to the phase error in said frequencysweep, said pulse being a phase correction component in said correctionsignal,

F. a reference oscillator whose signal frequency is an integral multipleof said sampling frequency generafor frequency,

G. a timing network,

H. means responsive to a first signal from said timing network forresetting the frequency of said oscillator to the beginning of saidrange after the swept signal reaches the end of its said range,

I. means responsive to a second signal from said timing network forremoving said correction signal from said correction circuit prior tothe beginning of the frequency sweep,

J. a phase lock circuit connected to a synchronize the initial output ofsaid variable frequency generator to the output of said referenceoscillator in response to a first signal from said timing network, saidtiming network also controlling said waveform generator so as tocommence the frequency sweep after the output of said variable frequencygenerator has been syn- ;zhronized to the frequency of said referenceoscil ator.

No references cited.

JOHN KOMINSKI, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,382,460 May 7, 1968 Daniel Blitz et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, equation (1), "f should read f equation (2) "6+f t" shouldread 6 t line 39, "6 should read 6 equation (4) second occurrence shouldread Column 5, equation (21), should read equation (24) "A6" should readA6 Column 7, line 49, "conected" should read connected Column 11, line15, simplified" should read amplified Column 14, line 43, "ingnal"should read signal Signed and sealed this 18th day of November 1969.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

1. A CONTROLLED SWEEP FREQUENCY GENERATOR FOR PROVIDING AN OUTPUT WHOSEFREQUENCY VARIES AS A SELECTED FUNCTION OF TIME, SAID GENERATORCOMPRISING A. A VARIABLE FREQUENCY GENERATOR, B. MEANS FOR REPETITIVELYSAMPLING THE PHASE OF THE OUTPUT SIGNAL FROM SAID VARIABLE FREQUENCYGENERATOR TO DEVELOP ERROR SIGNALS WHOSE MAGNITUDE AND POLARITYCORRESPOND TO THE DIFFERENCE BETWEEN SAID PHASE AND THE PHASE OF AREFERENCE SIGNAL, C. A SAMPLING FREQUENCY GENERATOR FOR CONTROLLING THESAMPLING RATE OF SAID SAMPLING MEANS, SAID SAMPLING FREQUENCY GENERATORHAVING A FREQUENCY SUCH THAT WHEN THE OUTPUT SIGNAL FROM SAID VARIABLEFREQUENCY